Effects of CeO2 on phase transformation towards cordierite in MgO–Al2O3–SiO2 system

Effects of CeO2 on phase transformation towards cordierite in MgO–Al2O3–SiO2 system

October 2001 Materials Letters 51 Ž2001. 68–72 www.elsevier.comrlocatermatlet Effects of CeO 2 on phase transformation towards cordierite in MgO–Al ...

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October 2001

Materials Letters 51 Ž2001. 68–72 www.elsevier.comrlocatermatlet

Effects of CeO 2 on phase transformation towards cordierite in MgO–Al 2 O 3 –SiO 2 system Z.M. Shi ),1, K.M. Liang, S.R. Gu Laboratory of AdÕanced Materials, Department of Materials Science and Engineering, Tsinghua UniÕersity, 100084, Beijing, People’s Republic of China Received 28 February 2000; received in revised form 12 December 2000; accepted 20 December 2000

Abstract The properties of cordierite ceramic depend upon additives and phase transformation in sintering. XRD and DTA techniques are used to study the effects of CeO 2 upon the transformation towards cordierite. As the experimental results show, with an increase of CeO 2 content, the temperature drops at which cordierite forms through diffusion in solid state; when liquid has occurred, the crystallization temperature first drops with CeO 2 content and then rises, the lowest measured temperatures being at 2–4 wt.% of CeO 2 . The effect of CeO 2 can be associated with the alteration of ion diffusion. q 2001 Elsevier Science B.V. All rights reserved. PACS: 64.70.Kb; 83.80Pc Keywords: Cordierite; Phase transformation; Cerium oxide; CeO 2

1. Introduction Cordierite ŽMg 2 Al 4 Si 5 O 18 . ceramic has been widely used as a material in kiln furniture, carriers of purifying exhaust emission, filters for liquid at high temperature, and partial electronic components due to its high thermal shock resistance, low thermal expansion coefficient and dielectronic constant. In order to obtain ceramic of high density and cordierite content and excellent sintering property, glass crys-

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Corresponding author. E-mail address: [email protected] ŽZ.M. Shi.. 1 Also with Inner Mongolia Polytechnic University, 010062, Hohhot, China.

tallization w1,2x and Sol–Gel methods w3,4x have been developed in addition to fine-grinding of minerals w5–9x. However, sintering minerals such as oxide powders are predominately used in large-scale manufacturing, especially as structural materials. Generally, some flux as B 2 O 3 , Li 2 O, La 2 O 3 w9–11x, etc., are added to modify the properties of cordierite ceramic. The additives as such result in some changes in sintering and ultimately, properties of ceramics. Rare earth elements manifests themselves as highly active substances and their oxides are excellent flux for sintering of ceramics. CeO 2 is a common rare earth oxide with higher melting temperature. Its proper addition into AlN w12x, ZrO 2 w13x and SrTiO 3 w14x ceramics is to promote sintering, thus

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resulting in microstructure stability and desirable modification of ceramics. Zdaniewski w1x, Kim and Lee w2x investigated the behavior of glass crystallization in MgO–Al 2 O 3 –SiO 2 –CeO 2 system, who found that CeO 2 not only reduces the temperature of glass softening and delays the crystallization of mcordierite, but also moves m ™ a conversion to relatively high temperature zone. Galaknov et al. w4x prepared, by Sol–Gel method, cordierite ceramic containing Ce 4q. The result of their exploration indicates that addition of Ce decreases the formation temperature of m-cordierite, yet has little influence on m ™ a transformation. But the effect of CeO 2 upon the transformation towards cordierite in powder-sintering process has not been reported so far, which is the very area for the authors of this paper to explore. Cordierite with stoichiometric composition when being sintered presents undesirable properties in terms of comparatively high sintering temperature, low cordierite content and high porosity. In this work, we studied the effects of CeO 2 on phase transformation in sintering of oxide powders by means of X-ray diffraction ŽXRD. and thermal difference analysis ŽDTA. in order to improve the sintering process and the thermal and mechanical properties of cordierite ceramic.

3. Results The DTA result is shown in Fig. 1. In order to examine the rules of transformation, the samples are sintered at specified temperatures close to the peaks of the curve. As shown in the X-ray data in Fig. 2, only new phases at the individual temperatures are marked for clarity. Metastable phase spinel ŽMgAl 2 O4 . forms as a result of the diffusive reaction between MgO and Al 2 O 3 in solid state at 12008C. But the peaks corresponding to enstetite ŽMgSiO 3 ., forsterite ŽMg 2 SiO4 . and mullite ŽAl 6 Si 2 O13 . are never found. At 12508C, a-cordierite has occurred, but neither m nor b cordierite is observed. When comparing the states at 12008C and 12508C, respectively, the peak height of spinel is found to rise continuously as that of cordierite increases. This suggests that cordierite is directly synthesized by the diffusive reaction amongst MgO, Al 2 O 3 and SiO 2 rather than that between spinel and silica. In the meantime, SiO 2 converts from a hexagonal structure to tetragonal. Held 2 h at 13708C, the samples with none of CeO 2 or 2 wt.% of it respectively still present a certain amount of tetragonal SiO 2 . Addition of 2 wt.%, however, makes the amount of residual SiO 2 start to decrease and a weak scatter peak appears, indicating the presence of

2. Experimental procedures The oxides ŽAR. are used as starting materials in proportion to the stoichiometric composition of cordierite Ž13.4 wt.% MgO, 34.8 wt.% Al 2 O 3 and 51.8 wt.% SiO 2 .. CeO 2 is added in nominal composition of 2, 4, 6, 8 and 10 wt.% into the mixture. The powders are wet-ground in ball mill for 10 h, and then dried. The phase transformation is measured with a Dupont 2100 instrument equipped with a platinum crucible, with a reference sample of aAl 2 O 3 and a heating speed of 108Crmin. The samples for XRD tests are molded through half-dry compression by 70 MPa, followed by sintering at different temperatures. The phase analysis is performed by an X-ray powder diffractimeter ŽRigaku Drmax-2400X; copper K-a radiation; 40 kV; 120 mA; scanning speed: 48rmin..

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Fig. 1. DTA curves of the samples with different CeO 2 .

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Fig. 2. XRD patterns of the samples sintered at different temperature. q: hexagonal SiO 2 , a: MgO, –: Al 2 O 3 , v: CeO 2 , s: spinel, q: tetragonal SiO 2 , c: acordierite.

glass phase. This result reflects that the solid solubility of CeO 2 into cordierite approaches to a limit and that extra CeO 2 dissolves into the liquid. When the content of CeO 2 is 6 wt%, the metastable phases such as SiO 2 and spinel disappear completely. In terms of a comprehensive analysis of the DTA and XRD patterns, the endothermic peak at Point 1 Žin Fig. 1. is attributed to the allotropic conversion of SiO 2 from hexagonal to tetragonal structures, and the solid solution of MgO and Al 2 O 3 into SiO 2 , but not to that of spinel. For the latter, there is a weak exothermic reaction as shown in Fig. 3. In fact, the dissolution of MgO and Al 2 O 3 into SiO 2 results at the incipient stage where cordierite forms. Consequently, the solid solution of SiO 2 starts to turn into cordierite at its limit of solubility, just as is indicated by the exothermic peak at Point 2. The endothermic peak at Point 3 results from the formation of liquid phase; Point 4 symbolizes a weak exothermic reac-

tion of crystallizing a-cordierite from liquid. This process continues until no more cordierite crystallizes. Finally, cordierite dissolves at Point 5. The

Fig. 3. DTA curves of SiO 2 and Al 2 O 3 qMgO.

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Fig. 4. Variation in transformation temperature with CeO 2 content.

temperature corresponding to each point at the DTA curve is denoted with T1 , T2 , T3 , T4 and T5 . The variation in the transformation temperature along with various contents of CeO 2 is shown in Fig. 4. As the contents increase, T1 , T2 , T3 and T5 all drop gradually and tend to be stable whereas only T4 drops at first and then rises again. The lowest temperature ŽT4 . occurs with the amount ranging from 2 to 4 wt.%.

4. Discussion The physiochemical process of sintering from oxide powders to cordierite is summarized as follows. Ž1. By means of solid state diffusion at the early stage of sintering, MgO and Al 2 O 3 not only combine into spinel but also concentrate in SiO 2 ; the concentration eventually results in an allotropic transformation from the hexagonal structure to the tetragonal structure, which is looser than the former because the conversion ŽH ™ T. is endothermic. The looser structure in turn facilitates the dissolution of MgO and Al 2 O 3 into SiO 2 . Ultimately, the solid solution converts into cordierite. Ž2. The liquid having appeared, metastable phase begins to dissolve into liquid. The formation process of cordierite turns into crystallization directly from the liquid. CeO 2 affects both processes of solid diffusion and crystallization by modifying the ion diffusion. Under the condition of high temperatures and between

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Ce 2 O 3 and CeO 2 there occur non-stoichiometric oxides w15,16x, in which exist oxygen ions capable of free movement. The intensity of ionic field Ž Zrr 2 , where Z stands for valence and r ionic radius. characterizes the extent of appeal of cation to anion w17x. On the basis of the physical properties of the cations below w18x, the value of Zrr 2 Ž5.285. of Ce 4q is higher than that of Mg 2q Ž3.856., but lower than that of Al 3q Ž10.48. and much more so than Si 4q Ž25.0., when all are in octahedral coordination. Hence, the integration of Si`O turns out to be the most stable. In the MgO–Al 2 O 3 –SiO 2 –CeO 2 quaternary system, the combination of Mg`O and Al`O is weakened by the interaction of ionic Ce to the O 2y of MgO and Al 2 O 3 . The interdiffusion of Al 3q and Mg 2q results in spinel. While Ce less affects the bond of Si`O in tetrahedron ŽSiO4 ., it cannot modify the diffusion of Si 4q. This is why spinel is easier to be formed than the silicates such as enstetite, forsterite or mullite. In addition, CeO 2 can dissolve into crystalline spinel partly due to their same cubic structure. The substitution of Ce 4q for Mg 2q enhances the lattice energy of spinel, only to benefit the disintegration of spinel and the conversion to cordierite. In a word, CeO 2 accelerates the diffusion of ions and the phase transformation to cordierite in solid state. As is mentioned above, Ce 4q differs from 4q Si and Al 3q in ionic radii, valence and the intensity of ionic field. According to glass structure theory w19x, Ce 4q belongs to glass network modifiers. Although located at the interstitial point of glass network, Ce 4q also applies a strong attraction to O 2y in the network. Proper amount of Ce in liquid makes ionic diffusion easier so that, with Ce added, cordierite may nucleate and grow easily from liquid at lower temperatures than without its involvement. On the other hand, excessive addition tends to get the network compact and ceases to reduce the liquid viscosity, consequently may become resistant to the diffusion of Si 4q, Al 3q and Mg 2 5 . As a result, crystallization temperature rises again. The fact that no other crystalline phase containing Ce 4q but cordierite is found, shows that most of CeO 2 dissolves into liquid, with a little integrated into the lattice of cordierite w20x. Since the radii of Ce 4q and Mg 2q are quite equivalent, Ce 4 may substitute Mg 2q in Mg`O octahedron, or be in-

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serted into the channel of six-membered rings of the hexagonal structure. This assumption is subject to further investigation. 5. Conclusions Ž1. CeO 2 lowers the formation temperatures of cordierite in solid state. The contents being 2–4 wt.%, it can also bring about the lowest crystallization temperature of cordierite. Ž2. CeO 2 promotes the formation of metastable phases and the conversion towards cordierite, thus enhancing the content of cordierite. Ž3. The mechanism of action by CeO 2 during sintering is chiefly related to the ion diffusion of Mg 2q, Al 3q and Si 4q. CeO 2 improves the ionic diffusion in solid state. After liquid occurs, CeO 2 in low content can decrease the viscosity of liquid and facilitates cordierite crystallization. With an increase of Ce 4q content, the gathering of Ce 4q to glass network restrains the diffusion of ions and therefore has to bring the crystallization to a higher temperature. References w1x W. Zdaniewski, J. Am. Ceram. Soc. 58 Ž1975. 163. w2x B.H. Kim, K.H. Lee, J. Mater. Sci. 29 Ž1994. 6592.

w3x S. Komarneni, J. Sol–Gel Sci. Technol. 6 Ž1996. 127. w4x A.V. Galakhov, V.Ya. Shevchenko, A.A. Stebunov, Refractries 32 Ž1992. 286. w5x V.N. Anlsiferov, S.E. Porozova, S.N. Pesherenko, Ogneupory 10 Ž1997. 20. w6x S.H. Naga, E.H. Sallam, D.M. Ibrahim, Int. Ceram. Rev. 43 Ž1994. 243. w7x M. Nakahara, Y. Konda, K. Hamano, J. Ceram. Soc. Jpn. 107 Ž1999. 308. w8x N.N. Sampathkumar, A.M. Umarji, B.K. Chandrasekhar, Mater. Res. Bull. 30 Ž1995. 1107. w9x K. Sumi, Y. Kobayashi, E. Kato, J. Am. Ceram. Soc. 82 Ž1999. 783. w10x E.H. Sallam, M.A. Serry, S.M. Naga, J. Mater. Sci. 21 Ž1986. 3269. w11x T. Okamura, T. Kishino, Jpn. J. Appl. Phys. 37 Ž1998. 5360 ŽPart 9B.. w12x I. Haase, CFI, Ceramic Forum-IntrBerichte der DKG 70 Ž1993. 404. w13x K. Tsukuma, Am.Ceram. Soc. Bull. 65 Ž1986. 1386. w14x M.S. Xiao, C.J. Wang, L. Chen, C.J. Zhang, X.X. Wang, J. Rare Earths 13 Ž1995. 16. w15x K.A. Gschneidner Jr., L.R. Egring, Handbook on the Physics and Chemistry of Rare Earths, Non-Metallic Compounds, vol. 3, North-Holland, New York, 1979. w16x E.C. Subbarrao, W.E. Wallance, Science and Technology of Rare Earth Materials, Academic Press, New York, 1980. w17x M.J. Jackson, N. Barlow, B. Mills, J. Mater. Sci. Lett. 13 Ž1994. 1287. w18x R.D. Shannon, Acta Crystallogr. A32 Ž1976. 751. w19x A. Paul, Chemistry of Glasses, 2nd Ed., Chapman and Hall, London, 1990. w20x M.A. Montorsi, R. Delorenzo, E. Verne, Ceram. Int. 20 Ž1994. 353.